The present invention relates to grounding and shielding of electronic circuitry, and more particularly to the grounding of circuitry that incorporates stripline and similar technology.
With reference to FIGS. 1 and 2, a conventional stripline circuit module 10 has a board or substrate 11 that has internal conductive circuit traces 12 that intersect component cavities 13 of the substrate 11. Thin conductive metal ground planes 14 are applied to opposite sides of the substrate 11, the cavities 13 also forming openings in the ground planes 14. Electronic components 15 are inserted into the cavities 13, the components 15 being electrically connected by soldering leads 16 thereof to exposed ends of the traces 12 in the sides of the cavities 13, the components 15 also typically protruding from opposite sides of the substrate 11 through the ground planes 14. Metal cover plates 17 are fastened to the ground planes 14, cover cavities 18 being formed part way through the plates 17 for receiving the protruding portions of the components 15. An elastomeric or conductive spring 19 within each cover cavity 18 grounds the associated component 15 to the plate 17, which is also grounded to the connected ground plane 14 by contact therewith, thereby creating a component grounding path from the component 15 to the ground plane 14 as indicated by the arrow in FIG. 2.
One problem with the configuration of FIGS. 1 and 2 is that the component 15 does not ground directly to the ground plane 14, thereby degrading the component grounding at higher frequencies wherein the wavelength is of the same order as the length of the component grounding path. Also, the edges of the module 10 are exposed dielectric material which may radiate or conduct EMI to or from adjacent stripline boards or some other outside receiver or source.
FIGS. 3 and 4 further show foil tabs 20 that are soldered to the ground plane 14 next to the components 15, the foil tabs 20 being folded respectively over the top and bottom of the components 14. The springs 19 in the cover cavities 18 press the tabs 20 against the components 15 such that the length of the grounding path from the component 15 to the ground plane 14 is greatly reduced. This configuration disadvantageously results in an air gap between the cover plate 17 and the ground plane 14, as shown at 21 in FIG. 4, the air gap 21 degrading the effectiveness of the ground plane 14 in the areas around the air gap 21, and causing EMI from one area of the module 10 to be transmitted to another area of the module 10 through the air gap 21. Further, because the foil tab 20 is soldered directly on the ground plane 14 which must remain as smooth as possible with evenly thick patches of solder, the soldering is critical and involves intensive labor.
To protect a stripline module against EMI from external sources, the edge of the substrate 11 must be sealed with metal. There are many traditional methods to edge seal stripline modules, but each method has the disadvantage of high cost and/or degraded performance.
Perhaps the best method to EMI shield a stripline board in the prior art is shown in FIG. 5. The substrate 11 is enclosed in a box 22 and lid 23, instead of cover plates. The box 22 provides superior protection against EMI due to its thickness, but costs much more than cover plates due to additional material and machining. The box 22 also increases the size of the module 10 significantly, which limits its application. The box 22 by itself does not degrade performance of the substrate 11, but if it is used in conjunction with the foil tabs 20 of FIGS. 3 and 4 for improved component grounding, the performance is degraded due to the air gaps 21 as described above.
FIGS. 6 and 7 show a common method to EMI shield a stripline module. A foil strip 24 is soldered to one ground plane 14, wrapped around the substrate 11 and soldered to the other ground plane 14. This "foil wrap" method induces air gaps 25 between the cover plates 17 and the ground planes 14 which cause the same problems as the foil tab induced air gaps 21, discussed above in connection with FIGS. 3 and 4. Like the foil tabs 20, the foil strips 24 are soldered directly on the ground plane 14, which must remain as smooth as possible and have no uneven patches of solder. Therefore, this soldering is critical and labor intensive. One way to eliminate the air gaps 25 associated with the foil wrap method is to machine a shallow channel along the edges of the cover plates 17 to accommodate the thickness of the foil strip 24. This, of course, adds additional cost to the cover plates 17 due to the additional machining and does not eliminate the air gaps 21 if the foil tabs 20 are used to ground the components 15.
Another method to EMI shield stripline substrates is to plate metal on the edge of the substrate 11. As shown in FIGS. 8 and 9, an edge plating 26 is formed along the perimeter of the substrate 11. The plating process and associated masking, however, is expensive. Also, edge plating by itself does not degrade the performance of the substrate 11, but if it is used in conjunction with the foil tabs 20, the performance is degraded due to the air gaps 21, discussed above.
Other methods for EMI shielding of stripline boards include painting the edge of the substrate 11 with conductive paint, inserting eyelets around the edge of the substrate 11, or inserting a conductive gasket around the edge of the substrate 11. These methods, due to size constraints, cost, or performance, are not commonly used.
Thus there is a need for a stripline shielding and grounding system that does not exhibit the above disadvantages, that is reliable, effective, and inexpensive to use.